Benefits from using Docker

As we have said before, the major visible benefit of using Docker will be very fast performance and short provisioning time. You can create or destroy containers quickly and easily. Containers share resources such as the operating system's kernel and the needed libraries efficiently with other Docker containers. Because of that, multiple versions of an application running in containers will be very lightweight. The result is faster deployment, easier migration, and startup times.

Docker can be especially useful when deploying Java microservices. We will get back to microservices in detail in one of the coming chapters. A microservices application is composed of a series of discrete services, communicating with others via an API. Microservices break an app into a large number of small processes. They are the opposite of the monolithic applications, which run all operations as a single process or a set of large processes.

Using Docker containers enables you to deploy ready-to-run software, which is portable and extremely easy to distribute. Your containerized application simply runs within its container; there's no need for installation. The lack of an installation process has a huge advantage; it eliminates problems such as software and library conflicts or even driver compatibility issues. Docker containers are portable; they can be run from anywhere: your local machine, a remote server, and private or public cloud. All major cloud computing providers, such as Amazon Web Services (AWS) and Google's compute platform support Docker now. A container running on, let's say, an Amazon EC2 instance, can easily be transferred to some other environment, achieving exactly the same consistency and functionality. The additional level of abstraction Docker provides on the top of your infrastructure layer is an indispensable feature. Developers can create the software without worrying about the platform it will later be run on. Docker has the same promise as Java; write once, run anywhere; except instead of code, you configure your server exactly the way you want it (by picking the operating system, tuning the configuration files, installing dependencies) and you can be certain that your server template will run exactly the same on any host that runs Docker.

Because of Docker's reproducible build environment, it's particularly well suited for testing, especially in your continuous integration or continuous delivery flow. You can quickly boot up identical environments to run the tests. And because the container images are all identical each time, you can distribute the workload and run tests in parallel without a problem. Developers can run the same image on their machine that will be run in production later, which again has a huge advantage in testing.

The use of Docker containers speeds up continuous integration. There are no more endless build-test-deploy cycles; Docker containers ensure that applications run identically in development, test, and production environments. The code grows over time and becomes more and more troublesome. That's why the idea of an immutable infrastructure becomes more and more popular nowadays and the concept of containerization has become so popular. By putting your Java applications into containers, you can simplify the process of deployment and scaling. By having a lightweight Docker host that needs almost no configuration management, you manage your applications simply by deploying and redeploying containers to the host. And again, because the containers are very lightweight, it takes only seconds.

We have been talking a lot about images and containers, without getting much into the details. Let's do it now and see what Docker images and containers are.